I start my period after I have sex with my husband every time!

Lately I have been starting my period after having sex with my husband no matter when I had my period last or how many times a week we have sex. It's not just bleeding during sex, it's a light spotting period for at least 3 days. I'm a mother of a 16 mo. old and just recently, 6 mo. ago, got off of the depo. I also use a Contraceptive gel called Ortho Options conceptrol. My husband and I usualy only have sex once a week and every time I start my period. I have yet to have a period like I did before I got pregnant, but all the problems seemed to have started when I started useing the Contraceptive gel. Also I am still breastfeeding. Does anybody know whats going on with me?